One of the things that got me so excited when I returned to playing RPGs was that there was - and still is - so much creativity in the OSR-space. What the OSR has been very good at is producing material that can be used directly at the table. That creativity spans adventures (for example, the Lamentations of the Flame Princess adventures, the Hill Cantons stuff), settings (the Midderlands, a Red and Pleasant Land, Yoon-Suin, etc.) and tools such as the Sine Nomine sandbox 'machinery'. And that last 'name-drop' is so important - the foregrounding of the player-directed exploration game - the sandbox - over the GM-directed plotted game is one of the strengths of the OSR, to my mind. Of course, the OSR isn't immune to Sturgeon's Law - there is a lot of dross - but there is so much absolutely excellent stuff that made use of the *common language* of class, level, HP, AC, etc.
But that doesn't mean that OSR stuff need be played using a TSR-D&D inspired 'class and level' system, does it? Now, I am a MASSIVE fan of the TSR-D&D game engine, so why might I want to use a different system? Depends on the mood and circumstances. Perhaps I want a crunchier, grittier game - in my stable of RPGs that'd involve using a BRP-based game or WFRP. Perhaps I have fewer players and want a game that can sidestep the expectation that the adventuring unit is a party with a spread a classes - in which case, as well as the above, my shelves might suggest Advanced Fighting Fantasy or perhaps even Barbarians of Lemuria. Perhaps the nature of our gaming group means that the slow level progression of TSR-D&D, or conversely, issues of substantial power increases as PCs move through the levels means that we need something with a flatter 'power curve'. Or perhaps, as GM, I just want to try out some nifty game system that isn't particularly new, but is new to me - such as Fate, PDQ, or Heroquest.
But I want to have my cake and eat it, obviously.
Now, I've run TSR-D&D/OSR material with only light adaptation using BRP (Magic World, in particular) and AFF. I know that people run 'Expert' tier (levels 4-12) D&D modules using Barbarians of Lemuria. I know that there is an amazingly thorough adaptation of Night's Dark Terror to WFRP1e over at Awesome Lies. So I know using OSR material with other systems can be done successfully: I've done this with some success, I know that you've done it with some success. But I would love to hear what people have done in this vein - what systems have you used, how did this change the game at the table, what worked, what failed, and how successful have been your attempts to use systems that diverge from the 'trad' assumptions within which I have almost exclusively gamed?